In submarine cable systems, submarine branching apparatuses are used for branching a submarine cable into a plurality of lines. In general, submarine branching apparatuses and submarine repeaters installed halfway through submarine cables are powered by constant current power source of land stations.
FIG. 6 is a block diagram illustrating a configuration of a common submarine cable system 900. The submarine cable system 900 includes a trunk station 1, a trunk station 2, a branch station 3 and a submarine branching apparatus 90. The trunk stations 1, 2 and the branch station 3 are land stations. The submarine branching apparatus 90 is installed on a seabed. The trunk stations 1, 2 and the branch station 3 are connected with the submarine branching apparatus 90 by submarine cables 51 to 53, respectively. The submarine cables 51 to 53 respectively include submarine repeaters 41 to 43 installed at certain intervals for amplifying transmitted signals and include transmission lines for transmitting signals and feed lines for supplying power to the submarine repeaters 41 to 43 and other apparatuses installed on the seabed. In many cases, metal conductor is used for the feed lines of the submarine cables 51 to 53 and optical fiber is used for the transmission lines.
The submarine branching apparatus 90 includes a branching unit 94. The branching unit 94 has a function for switching feed lines of the submarine cables 51 to 53 and a function for connecting the feed lines of the submarine cables 51 to 53 to sea earth for grounding. The submarine cable system 900 also includes a function for switching paths of transmitted signals. In the following, the function of the submarine cable system 900 for switching feed lines will be described.
The trunk station 1 illustrated in FIG. 6 has a constant current source of a positive voltage (positive supply) while the trunk station 2 has a constant current source of a negative voltage (negative supply). The feed current A from the trunk station 1 flows through the submarine cable 51, the submarine branching apparatus 90, and the submarine cable 52 to the trunk station 2. The branch station 3 is a constant current source of a positive voltage (positive supply), and the feed current B from the branch station 3 flows from the branch station 3 through the submarine cable 53 toward the submarine branching apparatus 90, and at the branching unit 94 the feed line is connected to sea earth.
FIG. 7 is a block diagram illustrating the feed lines in the case when the submarine cable 51 of the submarine cable system 900 in FIG. 6 is cut off at the position denoted by X. Upon detecting the fault in the submarine cable 51, the branching unit 94 connects the end of the submarine cable 51 connected with the submarine branching apparatus 90 to sea earth. The branching unit 94 connects the feed line of the submarine cable 52 with the feed line of the submarine cable 53. As a result, the feed current from the branch station 3 flows through the submarine cable 53, the submarine branching apparatus 90, and the submarine cable 52 to the trunk station 2. Since the trunk station 2 and the branch station 3 supply power by constant current, the feed current B from the branch station 3 needs to be equal to the feed current A from the trunk station 2 in the feed lines illustrated in FIG. 7. Supplying power to the submarine cables 52, 53 allows communication between the trunk station 2 and the branch station 3 to be maintained.
In relation to the present invention, Patent Literature 1 and Patent Literature 2 describe techniques for switching feed lines.